S1→Sn and S2→Sn absorption of azulene:: Femtosecond transient spectra and excited state calculations

The transient spectra of azulene in solution have been measured in the spectral region 600-350 nm at room temperature, pumping into the S-1 and S-2 states with femtosecond pulses and probing with a delayed femtosecond white light continumm. The spectra are a combination of ground-state bleaching, stimulated emission, and excited-state absorption. The latter component gives a direct information on excited states which may be not active in the ground-state absorption. S-1 --> S-2 and S-2 --> S-n absorptions have been discussed with the help of ab initio calculations of the MCSCF/CAS type with the 6-31G* basis set and including perturbative corrections. The calculated S-0 --> S-n, S-1,S-eq --> S-n, and S-2,S-eq --> S-n vertical excitation energies and oscillator strengths are in satisfactory agreement with the experimental results. It has been found that electronic states, weakly active in the ground-state absorption, occur with high intensity in the femtosecond transient spectra, in particular in the energy range 36000-44000 cm(-1) above the ground-state energy.